The physical constant which is less for $H_{2}O$ the $D_{2}O$ is

The physical constant, ___________ is less for $D_2$O as compared to that of $H_2$O.

The physical constant, ___________ is less for $D_2$O as compared to that of $H_2$O.

How can $D_{2}O$ be prepared from water? Mention the physical properties in which D_2O differs from $H_{2}O.$ Give at least three reactions of $D_{2}O$ showing the exchange of hydrogen with deuterium.